As a technique to improve the response speed of a display panel included in an image display device, overdrive drive (also called overshoot drive) is conventionally known. In conventional typical overdrive drive, when the gradation value of a pixel included in a video signal is changed to a high (low) value, a voltage higher (lower) than a voltage required to obtain a desired luminance (a luminance corresponding to the gradation value after change) is applied to a pixel circuit in a display panel in the first frame period occurring after the change of the gradation value. The following discusses a liquid crystal display device as an example of an image display device.
In a liquid crystal display device that does not perform overdrive drive, when the drive voltage of a pixel is changed as shown in FIG. 9A, the luminance of the pixel changes as shown in FIG. 9B. As shown in FIGS. 9A and 9B, when the drive voltage is changed with the change of a gradation value (not shown), the luminance changes slowly and thus it may take several frame periods for the luminance to reach a desired level.
On the other hand, in a liquid crystal display device that performs overdrive drive, the drive voltage of a pixel changes as shown in FIG. 10A, and the luminance of the pixel changes as shown in FIG. 10B. As shown in FIGS. 10A and 10B, in one frame period starting from time 0, the drive voltage gets higher than a level required to obtain a desired luminance and the luminance changes sharply and reaches a desired level in a short time. As such, by performing overdrive drive, the response speed of a liquid crystal panel can be improved.
Liquid crystal display devices performing overdrive drive are described in, for example, Patent Documents 1 to 3. Of them, Patent Document 2 describes a liquid crystal display device shown in FIG. 11. In FIG. 11, an emphasis conversion unit 93 obtains an emphasis conversion signal that compensates for the optical response characteristics of a liquid crystal display panel 94, based on an input image signal (a current frame image signal), an image signal of a previous frame period which is stored in a frame memory 91, and an image signal of a second previous frame period which is stored in a frame memory 92. The drive voltage of a pixel gets higher than a level required to obtain a desired luminance, over two frame periods occurring after the change of a gradation value (see FIG. 12A), and the luminance of the pixel changes as shown in FIG. 12B. By this, the occurrence of an after-image when performing moving image display is suppressed and a halftone can be displayed properly.